Hematologic cancers pose a unique challenge to the immune system, as they can present their antigens both directly to host T cells, or indirectly following transfer to host ARC, with fundamentally distinct signals accompanying each mode of presentation. For naive CD8+ T cells, the outcome of these events can determine whether an effective anti-tumor response is either induced, or rendered functionally silent through a variety of tolerance mechanisms. There are currently significant gaps in both our understanding of the antigen-presenting cells (APC) capacity of normal B cells or their transformed counterparts, lymphomas and myelomas, as well as the consequences of this for primary antigen-specific CD8+ T cells of in vivo. This application, based on a robust experimental model and supported by strong preliminary data, proposes the hypothesis that under physiological conditions, normal B cells efficiently program a durable state of tolerance in naTve CD8+ T cells through a novel mechanism involving transmission of signals that lead to primary expansion and subsequent uncoupling of the TCR proximal signaling machinery from the downstream cytotoxic effector functions in the clonal progeny. We further suggest that this same pathway is utilized by lymphomas to purge the available CD8+ T cell repertoire of clones capable of recognizing the multitude of antigens that arise during the process of transformation. The objective of this research is therefore to understand how normal and transformed primary B cells induce tolerance in CD8+ T cells under physiological conditions in vivo. Using a novel in vivo transgenic system that features expression of a model antigen on primary B cells, we will pursue the following aims: Aim 1) to characterize of the exact pathways in which B cells induce CD8+ T cell tolerance, Aim 2) to define the molecular defect underlying the tolerant phenotype of CDS T cells, Aim 3) to determine whether spontaneous lymphomas induce CD8+ T cell tolerance via the same pathway, and Aim 4) to establish methods through which the induction of CDS T cell tolerance by B cells can be prevented or overcome. The results of these studies will shed important new light on the APC function of normal and transformed B cells, and will allow the development of much-needed new strategies for enhancement of immune responses to B cell tumors.